Scars form in response to cutaneous injury as part of the natural wound healing process. Wound healing is a lengthy and continuous process, although it is typically recognized as occurring in stages. The process begins immediately after injury, with an inflammatory stage. During this stage, which typically lasts from two days to one week (depending on the wound), damaged tissues and foreign matter are removed from the wound. The proliferative stage occurs at a time after the inflammatory stage and is characterized by fibroblast proliferation and collagen and proteoglycan production. It is during the proliferative stage that the extracellular matrix is synthesized in order to provide structural integrity to the wound. The proliferative stage usually lasts about four days to several weeks, depending on the nature of the wound, and it is during this stage when hypertrophic scars usually form. The last stage is called the remodeling stage. During the remodeling stage the previously constructed and randomly organized matrix is remodeled into an organized structure that is highly cross-linked and aligned to increase mechanical strength.
Hypertrophic scar formation is a major clinical problem, which can give rise to exuberant scarring that results in permanent functional loss and the stigma of disfigurement. Clinical experience suggests that hypertrophic scarring is an aberrant form of the normal processes of wound healing. Hypertrophic scarring should be distinguished from keloid formation, the other major form of excessive scarring seen in humans. Keloids are characterized by overgrowth of fibrosis beyond the boundaries of the original injury, while hypertrophic scars do not extend beyond the original wound margins. Keloids and hypertrophic scars can also be differentiated by established histopathological criteria, which include differences in collagen density and orientation, vascularity, and other factors.
The pathophysiology of hypertrophic scar formation involves a constitutively active proliferative phase of wound healing. Scar tissue has a unique structural makeup that is highly vascular, with inflammatory cells and fibroblasts contributing to an abundant and disorganized matrix structure. The net result is that the original skin defect is replaced by a nonfunctional mass of tissue.
In both adult and fetal healing, the local wound environment interacts with the cellular components of wound healing and vice versa. The local wound environment consists of noncellular influences such as matrix components, oxygen tension, and mechanical forces. The interplay between cellular and noncellular components is complex, with constant feedback between the two during the healing process.
The inflammatory response is a normal component of the wound healing process, serving both as an immunological barrier from infection and as a stimulus for fibrosis to close the site of injury. Observations from human pathological specimens and from healing fetal wounds suggest that a robust inflammatory response may underlie the excessive fibrosis seen in hypertrophic scar formation. Mast cells, macrophages, and lymphocytes have all been implicated in this process. For example, mast cells have been shown to directly regulate stromal cell activity in vitro as well as to be strongly associated with the induction of fibrosis in vivo. Mechanical activity, age-specific changes, and delayed epithelialization have all been implicated as inciting factors for this intense inflammatory response.
A cell's external mechanical environment can trigger biological responses inside the cells and change cell behavior. Cells can sense and respond to changes in their mechanical environment using integrin, an integral membrane protein in the plasma membrane of cells, and intracellular pathways. The intracellular pathways are initiated by receptors attached to cell membranes and the cell membrane that can sense mechanical forces. For example, mechanical forces can induce secretion of cytokines, chemokines, growth factors, and other biologically active compounds that can increase or trigger the inflammatory response. Such secretions can act in the cells that secrete them (intracrine), on the cells that secrete them (autocrine), on cells surrounding the cells that secrete them (paracrine), or act at a distance from the point of secretion (endocrine). Intracrine interference can alter cell signaling, which can in turn alter cell behavior and biology including the recruitment of cells to the wound, proliferation of cells at the wound, and cell death in the wound. In addition, the extracellular matrix may be affected.
Following cutaneous injury, endothelial damage and platelet aggregation occur resulting in the secretion of cytokines including the transforming growth factor (TGF)-β family, platelet-derived growth factors (PDGF), and epidermal growth factors (EGF). These cytokines stimulate fibroblast proliferation and matrix secretion, and induce leukocyte recruitment. Leukocytes, in turn, reinforce fibroblast activity, fight infection, and increase vascular permeability and ingrowth. They do this acting through the TGF-β family, fibroblast growth factors (FGF), vascular endothelial growth factors (VEGF), and other factors. Prostaglandins and SMAD activation also increase inflammatory cell proliferation and impair matrix breakdown. Increased levels of TGF-β1 and β2 as well as decreased levels of TGF-β3 have been associated with hypertrophic scarring through inflammatory cell stimulation, fibroblast proliferation, adhesion, matrix production, and contraction. Consistent with these observations, anti-inflammatory agents (cytokine inhibitors, corticosteroids, interferon α and β, and methotrexate) have been used with some success to reduce scar formation.
Increased vascular density, extensive microvascular obstruction, and malformed vessels have also been observed in hypertrophic scars. These structural changes may account for the persistent high inflammatory cell density observed in hypertrophic scars. Conversely, persistent inflammation could itself contribute to increased vascularity through positive feedback loops.
Many cells are known to be mechanoresponsive. It has recently become clear that cells in the skin are also able to respond to their mechanical environment. Specifically, cell surface molecules such as the integrin family are activated by mechanical forces, leading to increased fibroblast survival as well as to the remodeling of deposited collagen and fibrin. Keratinocyte proliferation and migration are similarly regulated by mechanical stress. Following tissue injury, mechanotransduction may serve a biological function to signal the presence of a tissue defect. Cells experience the highest levels of mechanical stress on the edge of a monolayer and, in the same way, the wound margin experiences high levels of mechanical stress. These stresses may have evolved to stimulate components of wound healing and initiate repair. Differences in exogenous forces may act to change cellular activation in the wound healing milieu and, when overactivated, lead to hypertrophic scar formation. Skin subjected to high levels of stress (secondary to trauma or joint movement) usually demonstrates robust hypertrophic scar formation.
Methods of improving healing, particularly for amelioration of scarring, are of great interest. The present invention addresses this.